This invention relates to a method of reducing the turbidity of an aqueous chromic acid solution. Specifically, it relates to the treatment of chromic anhydride by adding of a small amount of water and heating, which reduces turbidity in a solution of the chromic anhydride.
In one process for producing chromic anhydride, sodium dichromate is reacted with excess sulfuric acid in water to produce a precipitate of the chromic anhydride: Na.sub.2 Cr.sub.2 O.sub.7 +H.sub.2 SO.sub.4 .fwdarw.CrO.sub.3 .dwnarw.. The chromic anhydride is collected and is heated to form a melt with the molten chromic anhydride on the bottom and sodium bisulfite and other impurities on top. The chromic anhydride is separated from the melt and is poured onto a water-cooled flaker roll. The melt solidifies on the flaker roll, forming a thin sheet which is removed by a doctor blade and is broken up into flakes.
When the flakes of chromic anhydride are dissolved in water, a solution of chromic acid forms: CrO.sub.3 +H.sub.2 O.fwdarw.H.sub.2 CrO.sub.4. Frequently, the chromic acid solution is turbid. Turbidity is undesirable because it affects the aesthetic qualities of aqueous products produced from chromic acid and, in turn, products produced from those solutions. While the turbidity seems to disappear after a period of time, it is not cost effective to permit the solution to sit for the time required for the desired reduction in turbidity.